The invention is directed to a method of making hardwood plywood, which is also known as decorative plywood. These terms are used interchangeably herein. Hardwood plywood is different from structural plywood. These differences have led to different manufacturing techniques for each type of plywood.
Both structural and hardwood plywood include a number of plies layered in a superimposed relationship. The direction of the grain of the wood in each internal ply runs transverse to that of the adjacent internal plies to give the panels significant lateral and longitudinal strength. However, structural plywood is used primarily for construction. Structural plywood commonly includes 4-7 plies. Its exterior or back and face plies, known as veneers, are rough sanded. The thicknesses of the back and face plies are not significantly different from the thicknesses of the internal plies. Scratches, defects and other minor errors in the back and face plies of structural plywood are typically not a significant concern because these structural plywood panels are normally used in application that are hidden in use.
In contrast, hardwood plywood must be aesthetically pleasing because it is visible in use, and is frequently used for the decor of a house or building. For example, hardwood plywood is used in cabinets, doors, and other articles. Due to its exposure, hardwood plywood must be essentially aesthetically perfect. As a result, panels having scratches, stains, or other minor imperfections are frequently considered to be defective. Beyond its appearance, hardwood plywood must also be strong and durable.
The primary structural difference between structural plywood and decorative plywood is in the face and back plies, the veneers. An example of a 5-ply hardwood panel 2 is illustrated in FIG. 1. The hardwood panel 2 includes a face veneer 4, a back veneer 10, and alternating core plies 6 and center plies 8. The face and back veneers 4 and 10 in hardwood plywood typically have a thickness in the range between 1/28-1/42 inch. These veneers are thin because of the cost associated with the type of woods used. The face and back veneers are usually made of oak, birch, maple, or other hardwoods.
The methods of manufacturing structural and hardwood plywood are also different. For example, structural plywood is significantly automated and is usually accomplished in a line process. In such an automated line process, the piece of veneer that will become the back of the panel is placed onto a conveyer and travels past additional stations. At each station, internal plies, referred to as core and center plies, are alternately added until a face ply is placed as the last layer of the panel. These superimposed plies are accumulated in some fashion to make a unit comprising of a number of panels. Most automated structural plywood assembly methods utilize sprayable resins, usually a phenol based product, for bonding adjacent plies together.
In contrast, the assembly of hardwood plywood has been much more labor intensive. Hardwood plywood frequently comes in 3, 5, and 7 ply panels. A back veneer panel is placed down. An internal ply, either a center or core ply, is placed on top of the back veneer. Subsequent internal plies are placed on top of a previous internal ply. The assembling and stacking of these plies are done by hand. The face veneer is placed on top of the uppermost internal ply and becomes the top of the panel. As previously discussed, the grain of the wood in each internal ply runs transverse to the internal plies immediately adjacent to it. Alternate internal plies, either the centers or the cores, are run through a standard glue spreader that applies glue to both sides. This process continues until a unit comprised of a number of stacked panels is formed. This unit is taken to a cold prepress and then a hot press so that the plies of the panels can be compressed and the glue can harden. This labor intensive process is also undesirable because it is time consuming. For example, it commonly takes 10-15 minutes to assemble a unit of 30 7-ply panels.
The automated line processing used in structural plywood has not been used for making hardwood plywood because of the thickness, and the fragile and aesthetic nature of the face and back veneers. Another reason that the automated line processing techniques are not used for hardwood plywood is that most hardwood veneers will oxidize and stain if placed in contact with metals and moisture. Yet another reason that automated assembly of hardwood plywood is not used is because phenol based resins, normally used in automated plywood processing, are unsuited for hardwood plywood production because of the tendency of the product to bleed through and oxidize defacing the veneers. The back veneers are not durable enough to carry the other layers in a similar fashion without damage, or by stains from oxidation or excess glue. Any or all of these problems can cause stains or other imperfections on the hardwood veneers which make the panels aesthetically defective and cause them to fall below the needed quality standards.
Therefore, a method of manufacturing hardwood plywood was thus needed which would reduce assembly cost through the reduction of time and labor. The present invention was developed to accomplish these and other objectives.